The second Robbins 5.6m diameter TBM has gone to face on the north drive of the $113M, 15.2km long Chattahoochee sewer tunnel in Cobb County, Tennessee. The start up in November 2001 sees both machines used on the tunnel now in action with the first Robbins having kicked-off boring on the south drive in August 2001.
The Gilbert/Healy JV is constructing the tunnel for the client Cobb County Water System. The two Robbins machines are tunnelling through various rock including gneiss, schist, quartzite and granite between 140-280MPa. Depths of 70m at the two launch shafts, one at the downstream end and one at the tunnel mid-point, and over 100m along the tunnel alignment have been chosen to optimise ground conditions. Rock support in zones of crushed and faulted rock will be taken care of by erecting steel sets immediately behind the cutterheads. Rock drills are being used to install rock dowels and for drilling pre-grouting injection holes for possible water inflow sealing. Spoil is being removed using a Robbins continuous conveyor system.
Tunnelling is scheduled to take a reasonably quick 16 months as half the tunnel is expected to remain unlined.
Upon completion the 0.1% downhill gradient sewer tunnel will feed the R.L. Sutton water reclamation facility near Chattahoochee River. The finished system is scheduled to come on-stream in October 2003.
